A Quenched and Relatively Isolated Dwarf Galaxy in the Local Volume

TL;DR

CVn C is presented as a rare isolated, quenched dwarf in the Local Volume, challenging the notion that current environment alone governs quenching in low-mass galaxies. Using deep HST imaging, the authors derive a TRGB distance of $8.43^{+0.47}_{-0.32}$ Mpc, an extended size ($R_e\approx1.1$ kpc) with low surface brightness, and a predominantly old stellar population with little to no star formation in the last $\sim$Gyr. Multi-pronged evidence, including CMD-fitting, archival FUV data, and HI non-detections (M_HI < $1.5\times10^6$ M_sun; M_HI/M_* < 0.44), supports a quenched, gas-poor state. The estimated isolation (tidal index $\Theta_5\approx -0.20$) and a nearby massive neighbor (NGC 4631) at ~1.3 Mpc, together with theoretical scenarios of past interactions or backsplash or cosmic web stripping, point to a complex mix of quenching pathways beyond present-day environment. These results contribute to defining the mass and environment regimes where different quenching mechanisms operate and motivate deeper kinematic and gas measurements for isolated dwarfs.

Abstract

An increasing number of discoveries of isolated and quenched dwarf galaxies are challenging the idea that the present-day local environment of low-mass systems is the main determinant of their quenching. We present new Hubble Space Telescope (HST) data of one such system, the dwarf galaxy Canes Venatici C (CVn C). CVn C is a low-mass (3.4(+4.2-2.6)*10^6 M_sun) galaxy with a Tip of the Red Giant Branch distance of 8.43(+0.47-0.32) Mpc determined from the resolved stars in the HST imaging, which we also use to derive CVn C's structural parameters. CVn C's distance places CVn C in the Local Volume and in an isolated environment with the most tidally influential L* galaxy > 5Rvir away. Additional constraints from the HST color-magnitude diagram, archival Far-Ultraviolet (FUV), and neutral hydrogen (HI) data show that CVn C is quenched, with no evidence of star formation in the last 100 Myr and no detectable gas (MHI < 1.5*10^6 M_sun). Circumstantial evidence suggests that CVn C may have quenched via past interactions with the L* galaxy NGC 4631 (L_K = 10^10.4 L_sun), and was possibly sent on an extreme backsplash orbit by the tidal dissolution of a subhalo group. However, other quenching mechanisms-such as stripping via the cosmic web-cannot be ruled out. CVn C adds to the growing number of quenched dwarf galaxies in under-dense environments, a population that will be critical to defining the mass and environment regimes in which different quenching mechanisms operate.

Figures (10)

• Figure 1: Color image of CVn C based on the ACS observations (left) and the WFC3 parallel field (right). A white $2R_e$ ellipse on the ACS image based on our measured structural parameters shows the location of CVn C. The F606W image for each instrument is colored blue, F814W is colored red, and the average of the F606W and F814W filters is shown in green. North is up and East is left. The image was made from the flc (charge transfer efficiency corrected) images of CVn C using the HST drizzlepac and APLpy python packages Hack2013Avila2015aplpy2012aplpy2019.
• Figure 2: Posterior Distributions of our measured structural parameters of CVn C based on fitting the spatial distribution of the resolved stars over 12,200 MCMC iterations. The $\Delta$RA and $\Delta$Dec values are the difference between the posterior distribution of CVn C's centroid and our initial by-eye estimate of the center of the distribution of well-recovered sources in CVn C. Vertical dashed lines in each histogram show the best-fit values and $1\sigma$ errors, while contours in each scatter plot show 1, 2, and 3$\sigma$ errors. Best-fit values for all parameters are reported in Table 1.
• Figure 3: The spatial distribution of sources passing our quality cuts. The overplotted ellipses are based on the structural parameters of CVn C. The inner $2R_e$ ellipse encompasses sources (shown in green) which are high-probability members of CVn C and are used for further analysis of the system. Stars outside the outer $3R_e$ ellipse (shown in red) are used as a measure of contamination. Sources in between $2-3\:R_e$, shown in grey, were not used in either analysis.
• Figure 4: The radial density profile of CVn C. A dot-dashed red line shows the Sérsic profile corresponding to the median structural parameters determined from our Bayesian inference approach. The red horizontal dashed line shows the uniform background density corresponding to the best-fitting $f_{bg}$, and the solid red line shows the sum of the Sérsic profile and uniform background. Grey lines show the same for 100 sets of parameters randomly sampled from the posteriors of our fit. Black points show the observed stellar density profile of CVn C and error bars show the corresponding Poissonian uncertainties. The observed stellar densities at large radii have been corrected for fraction of the corresponding area falling outside the ACS field of view.
• Figure 5: First Panel: CMD showing the 401 stars passing quality and spatial cuts in CVn C, with error bars showing the RMSE in three magnitude bins. The data points have been corrected for Galactic extinction based on the values listed in Table \ref{['tab:CVnC_prop']}. Second Panel: Hess diagram showing the binned density of stars in CVn C. Third Panel: Hess diagram showing the binned density of stars in the off-target ACS Field region used to identify background contamination. Both binned density diagrams are annotated with the area of the corresponding region. CVn C's galaxy region shows a clear RGB which is absent from the background field. Fourth Panel: Same CMD as first panel, now colored by the probability of the source being a contaminant. The probability is measured based on the proximity of the source in color-magnitude space to sources in the off-target ACS field region, as described in Section \ref{['sec:cmd']}. Sources brighter than or blue-ward of the RGB have a high probability of being foreground or background contaminants.